This application relates to LED (light-emitting diode) lamps, and in particular to an LED lamp to replace standard incandescent lamps.
Incandescent lamps have existed for over a hundred years. They are attractive and aesthetically pleasing for their high Color Rendering Index (CRI) and warm color temperature. However, low efficiency, short life and energy waste have been major drawbacks that forced consumers to switch to more efficient sources of light, such as the fluorescent lamp.
The incandescent lamp would have become obsolete many decades ago had it not been for the fact that fluorescent lamps have low CRI, are physically large, exhibit flicker effect and include hazardous materials such as mercury.
Until the late 1980's, LEDs had been primarily used as an indicator light in electronic equipment. Their high efficiency compared to incandescent made them very popular. Towards the late 1990's, high intensity LEDs started emerging, including the white LED. Today, the advancement in LED chip design and manufacturing makes it more feasible than ever to replace the incandescent lamp.
However there remain several challenges that slow the spread of LED lamps:
1. Low Maximum LED Junction Temperature and Heat Dissipation.
LEDs generate heat at a rate equal to the product of the voltage drop VD and the drive current ID,
      P          (      Watts      )        =                    I                  D          ⁡                      (            Amp            )                              ×              V                  D          ⁡                      (            Volt            )                                =                  Δ        ⁢                                  ⁢                  Q                      (            JOULES            )                                      Δ        ⁢                                  ⁢                  t                      (            Sec            )                              where P is the power and Q the heat energy produced by the LED. The LED junction temperature rise is a function of the difference between heat generated and heat dissipated. Heat dissipation is a function of the heat sink surface area, the thermal conductivity of the different media and interfaces and the temperature difference between the heat sink and ambient temperature. Most LEDs have a maximum junction temperature of 125° C. and a few manufacturers advertise up to 180° C. Light output from LEDs is limited by how fast heat can be dissipated away from the die. The luminous output of LEDs is reduced as the junction temperature elevates. FIG. 1 is a plot of the luminous output vs. junction temperature of a typical LED.
2. Luminous Output and Efficacy.
Luminous Efficacy is the ratio of luminous flux (Lm) to applied power (Watts). Typical values of low power incandescent lamp efficacy are:
Luminous EfficacyLM/WCombustion Candle0.3 5 W Incandescent540 W Incandescent12
LED efficacy has improved over the last few years and has exceeded 100 Lm/W. Commonly available power LEDs can measure up to 85 Lm/W. However, it should be noted that these measurements are taken at 25° C. junction temperature and reduced drive current.
As noted earlier, the luminous output decreases when the die temperature increases. Increasing the drive current has an even greater effect on reducing efficacy. As the current increases, the light output increases in a non-linear fashion, as shown in FIG. 2, but as FIG. 3 shows, the voltage increases as well.
In other words, if the current I is increased by a factor (1+K, where 0<K<1), not only will the luminous output be increased by a factor (k+1, where 0<k<K), but the LED voltage V will also increase by a factor (1+v). The new LED power consumption will become:P=(I+K)×(V+v)=(I+K)×V+(I+K)×v where the first term represents the increase in power due to increase in current only, and the second term represents the increase in power due to increase in current and voltage.
Thus, increasing the LED current will increase the LED output at the expense of reducing its efficacy. The percentage increase in lumens is lower than that of the increase in current, which will reduce efficacy at a much higher rate.
3. An LED is a Unidirectional Light Source.
LEDs emit light in a cone that is less than a half space, making it difficult to be used in a traditional “A” type lamp, as FIG. 4 shows. When mounted on a heat sink and placed in a bulb-like shell, some of the light will be absorbed by the package and lens material which will reduce the system efficacy. For a successful implementation of the LED in an “A” type lamp, the LED needs to be elevated to the center of the bulb, but this reduces the thermal dissipation capabilities.
4. Need for Power Conversion.
LEDs are current driven devices that require a constant current source power supply (FIG. 5). As FIG. 3 indicates, the voltage reflected by an LED is an exponential function of the drive current. An LED cannot be driven by a voltage source, since the source voltage must match the LED voltage. Otherwise, the difference in voltage divided by the total circuit resistance will result in a current that would easily exceed the maximum LED rating and cause the device to fail.
A constant current source power supply adds cost and reduces the reliability and efficiency of the LED lamp system. A fly-back power supply under 5 watts has a typical efficiency of less than 80% which will reduce the luminous efficacy of the whole lamp system.
Power supplies occupy valuable real estate in a lamp system, and special measures need to be taken in order to isolate a power supply from the heat generated by the LED.
5. Dimmability.
A dimmer controls the light output by phase controlling the AC input voltage. However, a constant current power supply will compensate for any change in input voltage in order to keep the output current constant. There are specialty power supplies that permit dimmability. These power supplies are designed to produce an output current that is proportional to the RMS input voltage. Such power supplies are generally more complex and exhibit lower efficiency.
FIG. 5 is a block diagram of an off-line switch mode power supply. A switch mode power supply is required to convert the 120Vac line voltage to a low DC current (10 mA-350 mA). Power supplies of 5 Watts output power or less are almost always fly-back type, and have a typical efficiency of less than 80%. They are also prone to failure if they encounter a surge, where a spike of high voltage could damage the MOSFET switch, especially if a surge suppressor, such as an MOV (metal oxide varistor), is not incorporated.
Another common method of driving low voltage LEDs is by using an impedance in series with AC line to limit the current and drop the excess voltage across it. This impedance may be a resistor, a capacitor or an inductor. A resistor is the cheapest and most available, but the energy E=I2.R.Δt it dissipates is lost and cannot be recovered. The losses increase with the increase in the difference in voltage between the source voltage and the LED voltage, as demonstrated in FIG. 6.                Assume Vs=166V, V=36V, and I=20 mA. Then,        
  R  =                              V          s                -        V            I        =                            166          -          36                0.02            =              6.5        ⁢                                  ⁢        k        ⁢                                  ⁢        Ω            
The power dissipation across the resistor is PR=I2×R=2.6W
The efficiency of the system becomes
            P      OUT              P      IN        =                    I        ×        V                              (                      I            ×            V                    )                +                  P          R                      =                            0.02          ×          36                                      (                          0.02              ×              36                        )                    +          2.6                    =                        0.72          3.32                =                  22          ⁢          %                    
Obviously this system is not feasible.
Another solution is to replace R by an impedance that does not dissipate energy, such as an inductor or a capacitor. A capacitor is more available in size and value than an inductor. The only limiting factor is the maximum allowable voltage drop across the capacitor. However, this solution renders the LED non-dimmable and increases the size of the circuit board due to the large size of the AC capacitor which needs to be rated to the line voltage plus a margin.
The resistor impedance solution would be feasible if the power dissipation is reduced, which is accomplished if the voltage difference (Vs−V) is reduced. This is done by increasing the number of LEDs in series until the total LED voltage drop approaches the source voltage Vs which will reduce the voltage difference (Vs−V) as well as the value of R required to limit the current.
For example, assume several LEDs are connected in series to produce a total load voltage V=136V. The new value for R is
  R  =                              V          s                -        V            I        =                            166          -          136                0.02            =              1.5        ⁢                                  ⁢        k        ⁢                                  ⁢        Ω            
The new power dissipation isPR=I2LOADR=0.6W 
The new system efficiency becomes
            P      OUT              P      IN        =                    I        ×        V                              (                      I            ×            V                    )                +                  P          R                      =                            0.02          ×          136                                      (                          0.02              ×              116                        )                    +          0.6                    =                        2.72          3.32                =                  82          ⁢          %                    
Clearly, this is well within the acceptable range for power supply efficiency, which is accomplished by shifting more of the wasted power to useful power.